Spin-transfer (spin torque or STT) magnetization switching described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), has recently stimulated considerable interest due to its potential application for spintronic devices such as STT-MRAM on a gigabit scale. Recently, J-G. Zhu et al. described another spintronic device called a spin transfer oscillator in “Microwave Assisted Magnetic Recording”, IEEE Trans. on Magnetics, Vol. 44, No. 1, pp. 125-131 (2008) where a spin transfer momentum effect is relied upon to enable recording at a head field significantly below the medium coercivity in a perpendicular recording geometry.
Materials with PMA are of particular importance for magnetic and magnetic-optic recording applications. Spintronic devices with perpendicular magnetic anisotropy have an advantage over Magnetoresistive Random Access Memory (MRAM) devices based on in-plane anisotropy in that they can satisfy the thermal stability requirement but also have no limit of cell aspect ratio. As a result, spin valve structures based on PMA are capable of scaling for higher packing density which is a key challenge for future recording devices.
PMA materials have been considered for MAMR applications as described by J-G. Zhu et al. in “Microwave Assisted Magnetic Recording”, IEEE Trans. on Magn., Vol. 44, No. 1, pp. 125-131 (2008). A mechanism is proposed for recording at a head field significantly below the medium coercivity in a perpendicular recording geometry. FIG. 1 is taken from the aforementioned reference and shows an ac field assisted perpendicular head design. The upper caption 19 represents a perpendicular spin torque driven oscillator for generating a localized ac field in a microwave frequency regime and includes a bottom electrode 11a, top electrode 11b, perpendicular magnetized reference layer 12 (spin injection layer or SIL), metallic spacer 13, and oscillating stack 14. Oscillator stack 14 is made of a field generation layer (FGL) 14a and a layer with perpendicular anisotropy 14b having an easy axis 14c. The ac field generator in the upper caption 19 is rotated 90 degrees with respect to the lower part of the drawing where the device is positioned between a write pole 17 and a trailing shield 18. The writer moves across the surface of a magnetic media 16 that has a soft underlayer 15. The reference layer 12 provides for spin polarization of injected current (I). Layers 14a, 14b are ferromagnetically exchanged coupled. Improved materials for the SIL and FGL are needed as this technology matures. In particular, it is highly desirable to design a system whereby the spin injection layer (SIL) is stabilized with respect to oscillations in an adjacent field generation layer. The SIL serves as a reference layer which requires a stable magnetization direction while the FGL is free to oscillate between different magnetic orientations.
U.S. Patent Application 2009/0225465 discloses a FeCo bias layer formed on an opposite side of the SIL with respect to a spacer layer. The bias layer may serve as an electrode and exerts a stabilizing magnetic field on the SIL.
U.S. Pat. No. 7,593,193 discloses ferromagnetic CoFe and CoFeNi layers in configurations used to increase the magnetoresistive (MR) ratio in spin valve structures.
U.S. Pat. No. 7,450,350 teaches a CoFe/NiFe/CoFe reference layer.